Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285; and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180). Moreover, this system has advantages over silver halide photography: it is a dry system, it enables direct visualization from digital data, it makes reproduction simple, and the like.
In this dye diffusion transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
In such a recording method in dye diffusion transfer system, it has been known that it is important to make the image-receiving sheet have high heat insulation and cushion characteristics in order to give a favorable image (see, for example, “Joho Kiroku (Hard Copy) to Sono Zairyo no Shintenkai (Information Recording (Hard Copy) and New Development of Recording Materials)” published by Toray Research Center Inc., 1993, pp. 241-285 and “Printer Zairyo no Kaihatsu (Development of Printer Materials)” published by CMC Publishing Co., Ltd., 1995, p. 180).
Thus, in some cases, a composite support using a biaxial oriented (stretched) polyolefin film containing microvoids was used as a base material for the image-receiving sheet to make the sheet have more heat insulation and cushion characteristics (see, for example, U.S. Pat. No. 866,282 and JP-A-3-268998 (“JP-A” means unexamined published Japanese patent application)). However in this method, there was occasionally caused a problem that the image-receiving sheet was wrinkled or curled by shrinkage due to relaxation of the residual stress after stretching by the heat during printing or the heat during formation of the image-receiving layer.
As other known methods of making the image-receiving sheet show heat insulation and cushion characteristics, a method in which, for example, a foaming layer composed of a resin and a foaming agent (see, e.g., Japanese Patent No. 2541796) or a porous layer containing hollow polymer particles (see, e.g., Japanese Patent No. 2726040) each having high cushion characteristics is formed between the support and the receptor layer, is known. The methods have an advantage that it is possible to prevent the image-receiving sheet from wrinkling and curling that are often found in the method in which a composite support made of a biaxially-oriented polyolefin film containing microvoids is used, because a heat-insulating layer can be formed on a base material by coating according to the method. However, it is generally difficult to produce a uniform smooth image-receiving sheet often causing problems such as poor image-transfer or transfer failure owing to insufficient contact between the ink sheet and the image-receiving sheet.
To solve the problems described above, an image-receiving sheet having a heat insulation layer made of hollow polymer particles and an organic solvent-resistant polymer as principal components is disclosed (see, e.g., Japanese Patent No. 3226167). However, the image-receiving sheet has not met a sufficient level. In addition, a method in which a solution for forming an intermediate layer is coated on a sheet-shaped base material and an image-receiving sheet is formed while pressing the coated face to a cast drum in forming an intermediate layer of a resin containing hollow particles as the principal component on the sheet-shaped base material, is disclosed (see, e.g., JP-A-5-8572). However, although such a method is effective in giving sufficient smoothness, it makes the production process more complicated and is thus disadvantageous from the viewpoint of productivity.
On the other hand, it is known, in the field of silver salt photography, that productivity can be greatly improved by simultaneously providing plural layers which have functions different from each other on a support through multi-layer coating of using aqueous coating solutions (see, for example, JP-A-63-54975 and Edgar B. Gutoff, et al., “Coating and Drying Defects: Troubleshooting Operating Problems”, John Wiley & Sons Company, 1995, pp. 101-103).
There is known a method of producing an image-receiving sheet using such the multilayer coating of using aqueous coating solutions (see, for example, JP-A-2006-88691). However, it is difficult to achieve sufficient cushion characteristics according to the method. Consequently, it causes transfer failure owing to insufficient contact between the ink sheet and the image-receiving sheet. This is because hollow polymer particles are so hard that it is difficult to achieve sufficient cushion characteristics, compared to the above-described “composite support made of a biaxially-oriented polyolefin film containing microvoids”.
It has been desired to improve a heat-sensitive transfer image-receiving sheet so that it causes no transfer failure and exhibits high cushion characteristics, and also to improve a method of producing the heat-sensitive transfer image-receiving sheet, by applying the multilayer coating technology using aqueous coating solutions that is preferable from technical viewpoints such as air pollution, fire hazard, sanitary working conditions as well as productivity, compared to the organic solvent based coating.